The Czochralski method (the CZ method) is known as a conventional method for growing a silicon single crystal, whereby a silicon single crystal of high purity for use as a semiconductor is grown from a silicon melt in a quartz crucible supported by a graphite crucible. This method is performed as follows. A seed crystal is mounted on a seed crystal holder, which is suspended via a wire in a chamber above crucibles from a rotation and pulling mechanism located at the top of a chamber. The wire is then drawn to bring the seed crystal into contact with the silicon melt. The seed crystal is pulled upward by the Dash necking method, the dislocation-free seeding method, or the like to prepare a neck portion from the silicon melt, and subsequently, the crystal is allowed to grow gradually to a target diameter. In this way, a dislocation-free single-crystal ingot having a desired plane orientation can be produced.
FIG. 2 shows a general example of the structure of a single-crystal production apparatus using a wire.
In the single-crystal production apparatus 50, a main chamber 20, which is composed of a top chamber 21, a middle chamber 22, a bottom chamber 23, and the like, houses crucibles 5, 6 that contain a raw material melt 4, a heat generator 7 for heating and melting the polycrystalline raw material, and the like. The crucibles 5, 6 are supported on a crucible-rotating shaft 19 that is capable of rotating and ascending or descending via a rotation drive mechanism not shown in the figure. The heat generator 7 is located to surround the crucibles 5, 6 to heat the raw material melt 4. On the exterior of the heat generator 7, a heat insulating member 8 is located to surround the heat generator 7 to prevent direct radiation of heat from the heat generator 7 into the main chamber 20.
Located at the top of the pull chamber 2 immediately above the main chamber 20 is a pulling mechanism 15 for pulling a grown single-crystal ingot 3. A pulling wire 16 is wound down from the pulling mechanism 15, and a seed holder 18 for mounting a seed crystal 17 is connected to the end of the wire. The seed crystal 17 mounted on the end of the seed holder 18 is immersed in the raw material melt 4, and the pulling wire 16 is wound up via the pulling mechanism. In this way, the single-crystal ingot 3 is pulled upward below the seed crystal 17 and grown.
For the purpose of, for example, discharging impurity gases produced inside the furnace from the furnace, an inert gas, such as argon gas or the like, is introduced into the pull chamber 2 and main chamber 20 via a gas inlet 10 located at an upper portion of the pull chamber 2. The inert gas passes along the single-crystal ingot 3 that is being pulled and passes over the melt 4, and circulates in the pull chamber 2 and main chamber 20, after which it is discharged via a gas outlet 9.
Each of the chambers has a coolant passage, which is not shown in the figure, and the coolant passage has an inside structure that allows a coolant for cooling the chamber to circulate, so as to protect each chamber, and block the radiation of heat from the heat generator 7 inside the chamber so that the heat is not transferred outside of the single-crystal production apparatus 50.
The pulling rate of the single-crystal ingot 3, i.e., the crystal growth rate, is determined according to the heat balance of the growing single crystal. The quantity of heat incorporated into the single crystal is classified into the quantity of inflow heat, which flows into the single crystal from the melt and heat generator; and latent heat of solidification, which is generated when the melt is crystallized. In consideration of the heat balance of the growing single crystal, the quantity of outflow heat, which is released out of the single crystal via the crystal surface and seed crystal, is equivalent to the sum of the quantity of inflow heat and latent heat of solidification. Latent heat of solidification depends upon the volume of the growth of the single crystal per unit time; hence, in order to increase the rate of crystal growth, it is necessary to compensate for the increment in the latent heat of solidification due to the increase in the crystal growth rate, by increasing the quantity of outflow heat.
In general, a method for increasing the quantity of outflow heat by effectively removing the heat released from the crystal surface is employed. For example, an apparatus has been suggested wherein a cooling cylinder 11, a cooling auxiliary member 13 that extends below the cooling cylinder 11, and the like are located inside the main chamber 20 to surround the single-crystal ingot 3 being pulled, to effectively cool the single-crystal ingot 3 being pulled, thereby increasing the pulling rate. Such an apparatus is disclosed in, for example, Japanese Unexamined Patent Publication No. H6-211589. This apparatus includes a gas-flow-guide cooling cylinder having a dual structure of an outer cooling cylinder made of a metal and an inner cooling cylinder made of graphite or the like, which are located to concentrically surround a single-crystal ingot being pulled from the lower portion of the pull chamber toward the inside of the main chamber. This causes the heat produced in the inner cooling cylinder to be transferred outside via the outer cooling cylinder to reduce the temperature increase in the inner cooling cylinder, thereby improving the cooling efficiency for crystals.
Moreover, for effective cooling of a growing crystal, International publication No. WO01/057293, for example, discloses an apparatus wherein a coolant is circulated in a cooling cylinder 11 via a coolant inlet 12 to effect forced cooling.
Typically, cooled water is used as a coolant for use in cooling each of the chamber members and cooling cylinder. Since a large quantity of water is used, water that has been heated after passing through the inside of the coolant passage and cooling cylinder is cooled in a cooling apparatus, such as a cooling tower or the like, and then the cooled water is temporarily stored in a water tank and re-circulated.
Incidentally, in the production of silicon single crystals according to the Czochralski method, there are two types of point defects introduced at the crystal-growth interface; i.e., vacancies where a silicon atom is missing, and interstitial silicon due to the introduction of excess silicon atoms. The ratio of these two defects is determined according to the ratio of the crystal growth rate, V, relative to the temperature gradient near the solid-liquid interface, G, i.e., V/G. In the cooling process performed during crystal growth, the predominant point defects of these two point defects near the melting point cause an aggregation reaction. At this time, when vacancies are predominant, they are detected as void defects, so-called COPs (Crystal Originated Particles), FPDs (Flow Pattern Defects), or the like (the V-region). When the above-described vacancies and interstitial silicon are equal in quantity, they interact with each other to cause the pair-annihilation reaction, thus producing a silicon single crystal with an extremely small quantity of defects that are detectable in a crystal-quality inspection (the N-region). When interstitial silicon is predominant near the melting point, it manifests itself as dislocations (the I-region).
These silicon single crystals find their own applications, and all of them are required to have a stable crystal quality. In order to realize this, it is necessary to stabilize the ratio V/G of the crystal growth rate V relative to the temperature gradient G near the solid-liquid interface. To stabilize the ratio V/G of the crystal growth rate V relative to the temperature gradient G near the solid-liquid interface, it is necessary to correct the pulling rate and the like appropriately. In general, however, it is not easy to determine an appropriate amount of correction; in particular, in order to obtain crystals of uniform quality according to the multi-pulling method, wherein a plurality of crystals are produced in the same crucible, it is necessary to correct the pulling rate for each crystal to a lower rate, as required. Furthermore, this correction does not necessarily depend only upon the operating time; thus, it has been difficult to control the crystal quality by making corrections based on only the total operating time from the beginning of the crystal production.